Neural electrodes are used to detect neural activity and excitation signals on a nerve and/or to provide for electrical stimulation of the nerve and must enjoy sound reliable electrical contact with the nerve for the duration of the detection or stimulation period which can extend over months or years without exerting an excessively high pressure on the nerve or even damaging it.
In previous arrangements therefore the actual electrode surfaces are fitted in the form of metal foils to the inside surface of an elastically expandable. spiral or helical main body of plastic material, which after implantation elastically embraces the nerve and in that way provides for a uniform contact pressure with the nerve even in the event of vigorous movements on the part of the patient and even in the event of the nerve swelling. A further development in an arrangement of that kind is to be found for example in WO-A-90/03824.
Besides helical carriers which involve a relatively complicated procedure for fitting them to the nerve and which generally require a special tool for that purpose. carriers of different shapes have also been put forward. Thus US-A-5 487 756 describes a cylindrical elongate sleeve which is divided in the direction of its longitudinal axis and which therefore can be easily opened up and closed again, with a locking bar which is inserted into the sleeve after it has been fitted to the nerve in order securely to close the sleeve around the nerve. In this case the electrodes are formed directly on the carrier by a thin-film process or by galvanisation.
EP-B-O 610 301 describes an arrangement with a carrier of a specific configuration such as to embrace the nerve with two Y-shaped arms of a connecting portion which extends along the nerve The connecting portion and the arms consist of flat silicone material and in this case the electrodes are preferably formed from an iridium foil or band and the carrier body is molded thereon.
In terms of clinical application of arrangements of the above-indicated kinds. it was found that the assembly suffers from unsatisfactory long-term stability.